Fabrication of semiconductor interconnect structure

ABSTRACT

An etching process for selectively etching exposed metal surfaces of a substrate and forming a conductive capping layer over the metal surfaces is described. In some embodiments, the etching process involves oxidation of the exposed metal to form a metal oxide that is subsequently removed from the surface of the substrate. The exposed metal may be oxidized by using solutions containing oxidizing agents such as peroxides or by using oxidizing gases such as those containing oxygen or ozone. The metal oxide produced is then removed using suitable metal oxide etching agents such as glycine. The oxidation and etching may occur in the same solution. In other embodiments, the exposed metal is directly etched without forming a metal oxide. Suitable direct metal etching agents include any number of acidic solutions. The process allows for controlled oxidation and/or etching with reduced pitting. After the metal regions are etched and recessed in the substrate surface, a conductive capping layer is formed using electroless deposition over the recessed exposed metal regions.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation-in-part claiming priority U.S. patentapplication Ser. No. 11/586,394 filed Oct. 24, 2006 now U.S. Pat. No.7,531,463, titled “Method for Fabrication of Semiconductor InterconnectStructure” naming Daniel A. Koos et al. as inventors, which is acontinuation-in-part claiming priority from U.S. patent application Ser.No. 10/690,084 filed Oct. 20, 2003 now U.S. Pat. No. 7,338,908, titled“Method for Fabrication of Semiconductor Interconnect Structure withReduced Capacitance, Leakage Current, and Improved Breakdown Voltage,”naming Daniel A. Koos et al. as inventors. U.S. patent application Ser.No. 11/586,394 is also a continuation-in-part claiming priority fromU.S. patent application Ser. No. 10/742,006 filed Dec. 18, 2003 nowabandoned, titled “Two-phase Plating of Cobalt Barrier Layers” by StevenT. Mayer and Heung L. Park, which is a continuation-in-part applicationclaiming priority under 35 USC 120 from the above identified U.S. patentapplication Ser. No. 10/690,084 filed Oct. 20, 2003 now U.S. Pat. No.7,338,908. This application is related to U.S. application Ser. No.10/317,373, filed on Dec. 10, 2002, entitled “Nitridation OfElectrolessly Deposited Cobalt,” by Heung L. Park. Each of thesereferences is incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE INVENTION

This invention pertains to methods for producing an effective cappinglayer for metal lines in integrated circuits. More particularly, theinvention pertains to methods of selectively etching metal lines and/orvias before depositing a conductive capping layer over the metalsurfaces.

BACKGROUND

The dielectric breakdown voltage current associated with metalinterconnect structures is determined by the intrinsic properties of thedielectric material interspersed between the metal lines as well asextrinsic properties such as the distance between the metal lines.

As device feature sizes continue to shrink and the distance between themetals lines is reduced, it is important to control the spacing betweenthe lines. This means close attention to patterning of the structures,the deposition of the metal, planarization of the structure and anysubsequent processing. One must avoid any encroachment between adjacentlines during processing in order to preserve good electricalcharacteristics of the structure.

Current technology uses an inlaid metal structure where the metal linesare formed by depositing a dielectric, pattern transfer and etching oflines in the dielectric, and subsequently depositing metals into thetrenches by various means. A conformal copper barrier, such as Ta or TaNis typically deposited over the entire surface by a PEVCD (plasmaEnhanced Chemical Vapor Deposition) process. Typically, a copper seedlayer is deposited on top of this copper barrier layer. The recesses inthe structure are then filled by a “bottom-up” non-conformal platingoperation. Additional copper metal (an “overburden” of a thicknesstypically equal to slightly more than the thickness of the dielectriclayer) is plated so that large, low aspect ratio features (those notfilled by the non-conformal process) are filled with metal up to theplane of the dielectric. The overburden of the metal deposition may beremoved by chemical mechanical polishing (CMP), and the individual linesand vias are thereby isolated. This is a general description of theso-called “damascene” process flow.

The space between adjacent lines is determined by various featuresincluding (1) patterning and etching of the trenches into which themetal is deposited, (2) the resulting etch profile, and (3) the depth towhich the metals and dielectric are polished during CMP. Note that CMPdepth affects lines spacing only if the features are not completelyvertical. The typical sought-after result is to have all surfacetopography removed and a planar surface between the metal and dielectricsurfaces.

Following this planarization process, a layer of silicon nitride isdeposited to encapsulate the layers and serve as a barrier to metal(primarily copper) diffusion and an etch stop for subsequent layers.Because this layer has a relatively higher dielectric constant than thesurrounding low-k dielectric layer, it can add significantly to theoverall capacitance experienced by the lines and interconnects, therebyhaving a negative impact on performance. A more recent process, whichselectively deposits a metallic “capping” layer, is superior because ofa reduction in line resistance. It also limits the deleterious effectsof device electromigration (EM), which results from defect sites at themetal/dielectric interface.

The conductive capping layer can be deposited on the metal lines priorto the encapsulating dielectric by a spatially selective method, such aselectroless plating or selective CVD. These methods are typicallyisotropic in nature and result in lateral as well as vertical growth ofthe newly deposited film. Thus, the resulting conductive capping layermay laterally spread over the dielectric layer causing adjacent metallines to encroach one another. This gives rise to a deleterious effecton the leakage and breakdown voltage of the device.

A typical capping layer process includes the following processoperations: dielectric deposition, etch to form trenches and vias,conductive barrier deposition, metal deposition, planarization,selective conductive cap deposition, dielectric barrier deposition(optional), and dielectric deposition.

The lateral growth of the capping layer reduces the effective spacebetween the metal lines, reducing the extrinsic insulating property ofthe interspersed dielectric and resulting in an increase in the electricfield between the metal lines. What is therefore needed is a cappingmethod that solves the problems of low breakdown voltages and high lineleakage typically encountered with conductive barrier capper layers.

SUMMARY

The present invention addresses the problems identified above byproviding methods to create a capping layer which provides for aninterconnect structure with reduced capacitance, leakage current, andimproved breakdown voltage. It accomplishes this by etching metal fromthe exposed metal regions of the substrate to a position below the levelof the exposed dielectric and forming a capping layer on the etchedmetal portions of the substrate using electroless deposition techniques.In other embodiments, the capping layer is deposited by non-electrolesstechniques such as PVD followed by planarization, selective CVD, andselective deposition from a supercritical solution.

In some embodiments, the exposed metal of the substrate is copper or acopper alloy, such as exposed copper lines or vias in Damasceneprocesses. Any number of suitable metal-containing capping layermaterials can be used. In preferred embodiments, the capping layercomprises a refractory metal such as cobalt or an alloy of cobalt. Theexposed metal of the substrate is preferably etched to a position belowthe level of the exposed dielectric that is approximately equal to orlower than the target thickness of the capping layer. The capping layeris then deposited to approximately the target thickness, which is chosensuch that it is thick enough to act as a diffusion barrier but not toothick as to unnecessarily increase the resistance between conductivepaths on adjacent metallization layers.

The etching can be performed in any suitable manner. In someembodiments, the exposed metal is contacted with an etching solutionthat oxidizes a portion of the exposed metal to a metal oxide. The metaloxide metal is then removed from the surface of the substrate using ametal oxide etching agent. In other embodiments, the exposed metal iscontacted with an etching solution that directly etches the exposedmetal without producing an insoluble metal oxide. In yet otherembodiments, the exposed metal is contacted with an oxidizing gas toproduce a metal oxide and the metal oxide is then removed from thesurface of the substrate using a metal oxide etching agent. An optionalannealing process can be used to pretreat the exposed metal surfacebefore etching.

In embodiments in which the exposed metal is contacted with an oxidizingetching solution, exemplary oxidizing agents include, but are notlimited to, peroxides, permanganates, persulfates, and ozone solutions,preferably at a pH of at least about 5. In addition to the oxidizingagent, the etching solution may contain a corrosion inhibitor tominimize grain attacks and surface roughening of the exposed metal.Alternatively, the exposed metal may be treated with corrosion inhibitorprior etching. In addition to the oxidizing agent, the etching solutionmay contain a complexing agent to control the etching rate and/or asurfactant to further modulate the etch rate.

Once metal oxides are formed on the surface of the substrate, they canbe removed by using any suitable technique. In some embodiments, themetal oxide is removed by using an oxide etching agent such as glycine,although any of a number of copper complexing agents may be used. Insome cases, metal oxide formation and removal take place in a singlesolution.

In embodiments in which the exposed metal is contacted with a directetchant and no metal oxide is formed, the etching solution may be arelatively high pH solution (e.g., a solution of tetramethyl ammoniumhydroxide, ethanol amine, ammonium hydroxide and the like). In addition,the direct etching solution may contain a corrosion inhibitor, acomplexing agent and/or a surfactant to further modulate the etch rate.

Any number of suitable techniques may be used to contact the substratesurface and exposed metal with the etching solution. These techniquescan include, but are not limited to, dipping, spraying or using a thinfilm reactor.

As indicated, the etching may comprise contacting the substrate with anoxidizing gas that oxidizes the exposed metal to a metal oxide and thentransferring the substrate to an aqueous solution containing a metaloxide etching agent to remove the metal oxide. The gas oxidation occursin a suitable reaction chamber. If the oxidizing gas is oxygen,preferred temperatures are between about 200 and 300 degrees Celsius andpreferable oxygen pressure is between about 50 and 180 Torr.

After the metal portions of the substrate are etched to provide newlyexposed recessed metal portions of the substrate, a post-etch treatmentmay optionally be performed prior to forming the capping layer. Ingeneral, post-etch treatments are used to promote better adhesion of thedeposited capping layer. Post-etch treatments can include those whichclean the newly exposed metal surface to remove remaining metal oxidesor other contaminants.

After the post-etch treatment, if implemented, a conductive cappinglayer is formed over the etched metal portions of the substrate usingelectroless deposition or other suitable technique. In some embodiments,the capping layer is formed using a two-phase method, which involvesfirst forming a metal nucleation layer on the etched metal portions ofthe substrate and then forming a bulk metal layer on the metalnucleation layer. In the first phase, the metal nucleation layer isformed by using an electroless deposition solution containing metal ions(e.g., cobalt ions) and a compound (e.g., water-soluble borane compound)that assists in nucleation of cobalt on a non-cobalt surface. In thesecond phase, a bulk metal layer is formed using a different electrolessdeposition solution containing metal ions and a compound (e.g., ahypophosphite reducing agent) that facilitates autocatalytic depositionof cobalt.

The process may also include a post-deposition anneal process which,under certain circumstances, allows for at least partial mixing ofdopants within the metal nucleation and bulk metal films. In otherembodiments, the process also allows for the optional formation of acobalt nitride film to further enhance the barrier properties of thecobalt capping layer. In these embodiments, the process further includesa nitriding operation to create a metal nitride layer on the bulk metallayer.

These and other features and advantages of the invention will bedescribed in more detail below, with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of cross section of a portion of aDamascene structure with capping layers formed by using traditionalmethods.

FIG. 1B is a schematic illustration of a cross section of a portion of aDamascene structure with capping layers formed by using methods inaccordance with the invention.

FIG. 2 is a flowchart summarizing a procedure for selectively etching ametal surface of an integrated circuit and depositing a capping layerover the metal in accordance with one embodiment of the presentinvention.

FIG. 3 is a graph comparing current leakage data of an integrated deviceusing traditional capping layer formation methods and an integrateddevice using capping layer formation methods of the present invention.

FIG. 4 is a flowchart summarizing a procedure for creating a cappinglayer over a copper metallization layer of an integrated circuit inaccordance with one embodiment of the present invention.

FIGS. 5A-5F are schematic illustrations of cross sections of a portionof an integrated circuit at different stages of development of a cappinglayer in accordance with embodiment of the present invention.

FIGS. 6A and 6B are schematic illustrations of a thin film reactorsuitable for the two stage electroless deposition methods of theinvention.

FIG. 7 is an experimental plot illustrating dependence of copper etchrate on glycine concentration at different concentrations of hydrogenperoxide.

FIG. 8 is an experimental plot illustrating dependence of copper etchrate on hydrogen peroxide concentration at different concentrations ofglycine.

FIG. 9 is an experimental plot illustrating dependence of copper etchrate on temperature for etching compositions containing glycine andhydrogen peroxide.

FIG. 10 is an experimental plot illustrating dependence of copper etchrate on pH of etching solution for etching compositions containingglycine and hydrogen peroxide.

FIG. 11 is an experimental plot illustrating dependence of reflectivityof etched copper layer as a function of thickness of the removed layerfor different etchant compositions and temperatures.

FIG. 12 is an experimental plot illustrating dependence of copper etchrate on pH for etching compositions containing different aminoacids andhydrogen peroxide.

FIG. 13 is an experimental bar graph showing copper etch rates fordifferent chemistries.

FIG. 14 is an experimental plot illustrating dependence of the amount ofetched copper on the spray etch time for an etching solution containingethylenediamine (EDA) and hydrogen peroxide.

FIG. 15 is an experimental plot illustrating pH dependence of the copperetch rate for an etching solution containing EDA and hydrogen peroxide.

FIG. 16 is an experimental plot illustrating dependence of the copperetch rate on hydrogen peroxide concentration for an etching solutioncontaining EDA and hydrogen peroxide.

FIG. 17 is an experimental plot illustrating dependence of the copperetch rate on EDA concentration for an etching solution containing EDAand hydrogen peroxide.

FIG. 18 is a bar graph illustrating copperetch rates for differentetchant compositions.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Introduction

As indicated, the present invention provides methods for selectivelyetching exposed metal from a substrate and forming a conductive cappinglayer on the etched exposed metal. The invention can be used to addressproblems associated with traditional methods of forming a capping layersuch as interconnect line encroachment which can lead to currentleakages within the interconnect and voltage breakdown of the device.Reference will be made to specific embodiments in accordance with thepresent invention. For instance, electroless deposition of cobaltcapping layers will be used as a principal example. While the inventionwill be described in conjunction with these specific embodiments, itwill be understood that it is not intended to limit the invention to anyparticular embodiment. A sampling of other deposition techniques andother capping layer materials will be provided below.

FIG. 1A depicts a cross sectional view of a Damascene device usingconventional methods for forming capping layers. Metal lines 101 areencapsulated with conductive barrier material 103 and surrounded bydielectric 105 such as silicon dioxide or a low-k material such as aporous silicon oxide and/or carbon containing silicon oxide. To preventthe occurrence of electromigration and to complete the encapsulation,capping layers 111 are selectively deposited over the metal lines 101.In some cases, a dielectric barrier layer 107 (e.g., silicon nitride) isadditionally deposited over the surface of the wafer on the cappinglayers 111. A second dielectric 109 of similar composition to dielectric105 is then deposited over the dielectric barrier layer 107.Subsequently, vias are etched through dielectric layers 107 and 109 toprovide interconnects for a next metallization layer.

As shown in FIG. 1A, the portions of the capping layers 111 extendingover the top of dielectric layer 105 spread on either side of the metallines 101 and taper to the same width as the metal lines 101 at the topof the capping layers 111. This mushroom shape is a result of usingtraditional electroless plating methods. These traditional methodsdeposit material in an isotropic fashion—that is they deposit materialin a lateral as well as a vertical direction. The lateral growth of thecapping layer reduces the distance 113 between the metal lines 101,thereby increasing in the electric field between the metal lines 101.

The present invention addresses this problem by incorporating processoperations that alter the Damascene structure prior to depositing acapping layer. Specifically, methods of the invention can be used forselectively etching exposed portions of metal lines, creating recessedregions of the surface of the substrate where the metal lines exist,followed by selectively forming the capping layer on the recessed metalregions. This process provides a structure in which the distancesbetween the conductive capping layers are substantially the same as thedistances between the metal lines and wherein the surface of thesubstrate is substantially horizontally planar. In other words, themushroom structures of FIG. 1A are replaced with the columnar structuresof FIG. 1B.

For example, FIG. 1B depicts a cross sectional view of a Damascenedevice with a capping layer produced using methods of the presentinvention. Metal lines 115 are encapsulated with conductive barriermaterial 117 and surrounded by dielectric 119. To prevent the occurrenceof electromigration and to complete encapsulation, capping layers 121are selectively deposited over the metal lines 115. As indicated above,a dielectric barrier layer 123 may additionally be deposited over thesurface of the wafer on the capping layers 115. A second bulk dielectric125 is then deposited over the dielectric barrier layer 123.

As shown in FIG. 1B, the capping layers 121 are of about the same widthas the metal lines 115. In addition, the top portions of the metal lines115 are recessed below the level of the top of the surroundingdielectric 119, thereby providing space for the capping layers 121 to bedeposited and resulting in a horizontally planar surface 129 at theinterface with the dielectric barrier layer 123. Compared to the methodsto form the structure of FIG. 1A, methods used to form the structure ofFIG. 1B avoid encroachment of the conductive capping layers into theinterspersed dielectric, thereby providing more robust insulation of theconductive regions of the interconnect structure and less electric fieldinteraction between metal lines.

Example Process Flow

A detailed process flow to provide the improved interconnect structuredescribed above will now be described. Note that this process flow is anexemplary embodiment and does not encompass the full range of possibleembodiments in accordance with the invention. For example, in theprocesses described below for formation of the capping layer, a detaileddescription for the deposition of a cobalt-containing capping layer isdescribed. It should be understood that the processes might also beapplied to the formation of other conductive capping layers such asthose that include palladium, ruthenium, platinum, tungsten, lead,cadmium, tantalum, tantalum nitride, nickel, titanium, titanium nitride,molybdenum, and combinations and alloys thereof. Some of these materialsmay be combined with relatively small amounts (up to about 10% byweight) of non-metallic compounds such as boron, phosphorus, carbon,silicon, nitrogen, and sulfur. In each case, the material should act asa barrier to the diffusion of copper.

Further, the invention extends to non-electroless deposition processessuch as (a) physical vapor deposition (PVD) followed by chemicalmechanical polishing or other planarization technique, (b) selectivechemical vapor deposition (CVD) on the etched metal regions, (c) atomiclayer deposition (ALD), (d) selective reduction of an organometallicprecursor from a supercritical solution including carbon dioxide forexample, and the like.

In preferred embodiments of the invention, the substrate is asemiconductor wafer containing partially fabricated integratedcircuitry. In the embodiments described below, methods for selectivelyforming capping layers on exposed copper surfaces surrounded bydielectric material will be disclosed. Such methods are useful inDamascene structures, for example, wherein capping layers are formed oncopper lines and/or vias. It should be noted, however, that methods ofthe invention might be implemented on any metal surface. For example,the metal surface may contain aluminum, tungsten, molybdenum or alloysthereof, as well as alloys of copper.

A typical process flow for the formation of a capping layer inaccordance with this invention is illustrated in the flowchart of FIG.2. Typically, though not necessarily, the process begins with aplanarized surface of the substrate. Any suitable planarizationtechnique, such as CMP or electroplanarization, may be used. In manycases, planarization will proceed to a point where all metal has beenremoved between the interconnecting lines (i.e. at a point where all thelines are electrically isolated). However, the process can also beginwith a surface that has not been planarized or has been partiallyplanarized to remove residual topography but not to the point ofexposing the underlying dielectric. In this case the etchant initiallyremoves the metal from the field area and above the lines at essentiallythe same rate as that over the metal until all of the copper is removedfrom over the field. Then the operation proceeds as in the generalsequence now described.

The first operation on the planarized and cleared substrate is anoptional pretreatment of the substrate as indicated at 201. One exampleof such pretreatment is an anneal, i.e., a thermal process that changesthe morphology of the copper and helps stabilize the crystal structureof the copper. This process serves to prevent the copper fromexperiencing significant mechanical stress in subsequent IC thermalprocesses. Preferable wafer temperatures of thermal anneal processes arebetween about 150 and 400 degrees Celsius. Temperatures in the higherend of this range are generally preferred, but may be unattainable forsome dielectric materials that are not able to handle thermal stress.The anneal time may range between about 20 seconds and 2 hours, withlonger times being required for lower temperatures. In one example, theanneal is performed in an inert atmosphere such as a forming gas (N₂/H₂)or under vacuum. In the case of forming gas anneals, a specific gascomposition of about 97% N₂ and about 3% H₂ is used. The gas flow may beabout 100 L/min dependent on the size of the chamber. In the case ofvacuum anneals the vacuum may be held at about 10⁻³ to 10⁻⁵ Torr.

Referring back to FIG. 2, once the appropriately pre-treated substrateis provided, the surfaces of the copper lines and/or vias areselectively etched (versus the dielectric and preferably both thebarrier lining and the dielectric) to form recessed copper surfaceswhere the lines and/or vias exist. This is achieved by selectivelyetching the exposed surfaces of the copper. See process block 203. Theamount of metal etched is preferably equal to or greater than the targetthickness of the conductive capping layer that will be deposited.

Note that if the method begins with a substrate in which the dielectricfield regions are not already exposed, the etching operation may proceedthrough the overlying copper (overburden) to expose the top of thedielectric layer and then continue from there a short distance below thenewly exposed field region of the dielectric. Note that this may beappropriate in a situation where a planarization technique other thanCMP is used (e.g., an electroplanarization technique) or whereplanarization is only partially completed.

Any suitable methods for etching the exposed metal of the substrate maybe may be employed to remove a portion of the exposed metal from thesurface of the substrate to produce a new exposed metal surface at aposition below the level of the dielectric and to provide space for asubsequently deposited capping layer. Methods that selectively etchmetal surfaces and do not substantially etch the surrounding dielectricare employed. Issues surrounding etching are described in U.S. Pat. No.5,486,234 issued Jan. 23, 1996 to Contolini, Mayer, and Tarte, which isincorporated herein by reference for all purposes.

It should be noted that this etch process, in addition to providingspace for the capping layer, can provide additional benefits toward theintegrity of the interconnect structure. In particular, after an etchprocess, the newly exposed copper surface is typically slightly rougherthan the original copper surface due the nature of etching processes,thereby potentially providing better adhesion of the capping layer tothe copper. Processes for etching the metal, however, should not roughenthe metal surface so much as to create pits or cavities deep enough toretain pockets of moisture during subsequent process operations.

Three general methods of selectively etching the exposed copper will nowbe described. Two of the methods involve a two-part process in which acopper oxide is formed followed by removal of the copper oxide from thesurface of the substrate—one in which the copper oxide is formed using awet chemical solution and one in which the copper oxide is formed usingan oxidizing gas. The third method involves directly etching andremoving the copper from the substrate surface in one step. Note thatdifferent etch techniques may produce different topologies on the metalsurface. For example, methods that directly etch copper often result ina rougher metal surface compared to methods that form a copper oxideintermediate. Further, methods that form an oxide tend to etch uniformlyacross the metal surface in a feature and are essentially independent offeature size. This is because the oxidation reaction is generally notdiffusion controlled, but is instead controlled by surface kinetics.

Any number of suitable techniques may be used to contact the substratesurface and exposed metal with the etching solution. These techniquescan include, but are not limited to, immersing, spraying, spin oncontact, and the like. In one example, apparatus for applying etchingsolution include those used for many EBR (edge bevel removal) or SRD(spin rinse drier) applications. An example of a suitable apparatus andmethods for its use are described further in U.S. Pat. No. 6,309,981 andin U.S. Pat. No. 6,586,342 issued Jul. 1, 2003 to Mayer et al., both ofwhich are incorporated herein by reference for all purposes. In apreferred approach, the etching solution is sprayed onto a rotatingsubstrate that is rotated between about 20-200 rpm at ambienttemperatures.

As indicated, one method involves indirectly etching by exposing theunderlying metal to an etching solution that oxidizes a portion of themetal to a metal oxide (e.g., copper oxide), followed by removal of themetal oxide from the surface of the substrate using a metal oxideetching agent. Any suitable oxidizing agent capable of forming copperoxide may be used, however, it is generally preferred that aself-limiting oxidation process be used. That is, the oxidation of thecopper occurs slowly and controllably. Exemplary oxidizing agentsinclude, for example, dilute aqueous solutions of peroxides (such ashydrogen peroxide), persulfates, ozone and/or permanganates. In someembodiments, the oxidizing solution has a relatively high pH, e.g., atleast about 5. In more specific cases, the solution has a pH of betweenabout 5 and 12, and in even more specific cases, between about 6 and 10.To control the oxidizing solution pH, a buffering agent may be used,preferably one with an anion that does not complex with copper ions.Examples include tetra-alkyl ammonium and alkali metal salts ofhydroxides. The oxidizing etch solution may also contain a complexingagent that complexes with the copper to control the etching rate of theacid, and/or a surfactant to further modulate the etch rate.

Generally, the copper oxidation process takes place uniformly over thefeatures of the partially fabricated IC. Thus, the etching isindependent of feature size, feature separation, position within afeature, etc. Again, this is because the oxidation rate is controlledprimarily by surface reaction kinetics, as opposed to diffusion ofcompounds to and/or from the copper surface.

Once copper oxide is formed by the oxidizing solution, it can be removedby using any suitable copper oxide etchant. In some embodiments thecopper oxide etchant selectively removes copper oxide withoutsubstantially attacking the copper crystallites or grain boundaries.Suitable copper oxide etchants include dilute acids, glycine and variouscopper complexing agents, for example. Exemplary acids includedissociated inorganic acids such as phosphoric acid, sulfuric acid andorganic acids such as acetic acid. Appropriate pH for the etchingsolution is typically in the range of about 0 and 2. Suitable complexingagents may include ethylenediamine tetraacetic acid (EDTA), citric acidand salts thereof, maleic acid and salts thereof, and certain ammoniumcompounds known to those of skill in the art, for example.

In some embodiments, separate oxidizing and oxide etching solutions areemployed. In other embodiments, a single solution is used for bothoxidizing copper and removing copper oxide. By controlling the ratio ofcopper oxidizing agent and copper oxide etchant in such solutions, onecan control the amount of oxidation and depth of the intermediate copperoxide film that is formed on the surface of the substrate. In apreferred embodiment, the solution includes between about 0.05% and 15%glycine (or copper complexing agent) by weight and between about 0.5%and 20% peroxide (e.g., H₂O₂) by weight. In a specific embodiment, forexample, an etching solution containing about 1% (by weight) glycine andabout 3% (by weight) H₂O₂ is used. Preferably, the single solutionincludes a buffering agent that maintains the pH at a specific value.Buffering agents such as acetate, carbonate, or phosphate can beselected depending on the desired pH value. More specifically, thesolution may have a pH of between about 5 and 12, and in even morespecific cases, between about 6 and 10. The pH can be adjusted by theaddition of an appropriate agent such as an alkali metal or tetra-alkylammonium hydroxide.

The etching and/or oxidizing solution may additionally contain acorrosion inhibitor to minimize grain attacks and surface roughening ofthe exposed copper metal. Suitable corrosion inhibitors include, but arenot limited to, benotriazole (BTA), thiourea, certain mercaptans, andimidazoles. Note that in addition to or instead of adding corrosioninhibitor to the etching solution, the substrate surface may be treatedwith a solution containing corrosion inhibitor prior to etching.

It should be noted that there exists a family of metal “polishing”solutions such as those used in metal CMP processes or mirror polishingprocesses that may be used in some embodiments of the present invention.These polishing solutions form an oxide and immediately etch it off thesubstrate. They are sometimes slurries that are sufficiently viscous toretard the transport of intermediate copper oxides (or otherintermediate species) away from the substrate, thereby slowing down theformation of more intermediate copper oxides and etching away of theseintermediate copper oxides. These methods provide very controlledetching so that smooth metal surfaces will result. As one example, the“polishing” solution may include ammonium cerium nitrate with nitricacid. Another example comprises inorganic substantially anhydrous acidswith small amounts (e.g., about 0.2 to 5%/wt) of oxidizers (e.g.,85%+/wt phosphoric acid in water together with hydrogen peroxide orsulfuric acid with ozone). Yet another example comprises organic acidswith small amounts of oxidizers (e.g., glacial acetic acid with about 1to 5%/wt of permanganate).

A second etching method involves exposing the copper to an etchingsolution that directly etches the exposed copper and removes the coppermetal from the substrate without producing an intermediate copper oxide.Chemicals that directly etch copper tend to preferentially etch at grainboundaries of the copper and roughening the copper surface. As describedpreviously, to some extent roughening of the copper surface may help topromote adhesion of the capping layer but too much roughening of thecopper surface can result in the development of pockets of moisture thatcan form copper oxide after the capping layer is deposited anddeteriorate the copper interconnect structure. Therefore, in preferredembodiments, the direct etch solution preferably provides a controlled,non-aggressive etch. The etching can be controlled for example bycontrolling the pH of the solution (acidic solutions tend to etch morequickly) and/or by including complexing agents to the etching solutionthat complex with the copper.

In one embodiment, a direct etch is accomplished using a relatively highpH solution such as a solution of TMAH (tetra methyl ammoniumhydroxide), ammonium hydroxide, ethanolamine, and the like. In otherembodiments, the direct etching solution has a weakly acidic compositionincluding, for example, citric acid, dilute sulfuric acid, etc.,together with one or more additives that control etch rate. Examples ofsuch additives include, but are not limited to, corrosion inhibitorssuch as benzotriazole and thiourea. Note that the etch rate can progressvery rapidly in acidic solutions because the reaction products aregenerally soluble and do not limit the reaction rate. The etchingsolution may also contain a complexing agent, corrosion inhibitor and/orsurfactant to further modulate the etch rate. Suitable complexingagents, corrosion inhibitors and surfactants may include those mentionedpreviously as well for indirect etching solutions. Any of a number ofdirect etching solutions may be used. A list of appropriate direct metaletching solutions can be found in “The Handbook of Metal Etchants”, CRCHandbook of Etchants for Metals and Metallic Compounds by Perrin Walker,William H. Tarn, Susan C Smolinske, Emma Previato (Contributor), ElenaMarchisotto (Contributor).

A third etching method involves exposing the copper to an oxidizing gasto produce a copper oxide, followed by transferring the substrate to asolution containing a copper oxide etching agent to remove the copperoxide. One of the advantages of using a gas oxidation process is thatthe amount of oxidation (i.e. depth of the copper oxide film) can bedirectly controlled by temperature of the reaction. The gas oxidationoccurs in any of a number of suitable reaction chambers such as aconventional CVD or plasma etch reactor. Any suitable oxidizing gas thatcan chemically react in a self-limiting fashion with the copper to forma passivating film on the copper surface may be used. If the oxidizinggas is oxygen, preferable temperatures are between about 200 and 300degrees Celsius and preferable oxygen pressure is between about 50 and180 Torr. Examples, of other oxidizing gases include sulfurhexafluoride, chlorine, and the like. Note that the oxidizing gas may bewholly or partially converted to a plasma in the etching chamber.Suitable copper oxide etchants include those mentioned previously suchas a dissociated acid. In a preferred process, the copper oxide isremoved using dilute sulfuric acid (pH<2).

Returning to FIG. 2, after the metal portions of the substrate areetched to provide newly exposed recessed metal portions of thesubstrate, a post-etch treatment may optionally be implemented prior toforming the capping layer. See process block 205. When the substrate isexposed to water and air in the ambient after the etch process, thecopper surfaces can readily oxidize to form a copper oxide film.Generally, this copper oxide film reduces adhesion of the subsequentlydeposited capping layer (although, to some extent it has been found thata little bit of copper oxide can aid to the adhesion). Post-etchtreatments may be employed to remove at least a portion of the copperoxide to promote better adhesion of the deposited capping layer.Frequently, this is done by a reduction of the copper oxide back tocopper metal using an acidic aqueous solution, typically having a pH ofabout 5 or lower, more preferably about 4 or lower. A dilute sulfuricacid and phosphoric acid have been found to work well. To address thepotential problem of copper ions redistributing into the dielectricduring the cleaning process, the cleaning solution preferably includescomplexing agent such as EDTA.

Returning again to FIG. 2, after the post-etch treatment, ifimplemented, the next operation forms a conductive capping layer overthe etched metal portions of the substrate using electroless deposition207. In the case of a cobalt capping layer, the deposition process mayemploy a solution of cobalt ions together with an appropriate reducingagent such as N,N-dimethylamine borane (DMAB) or a source ofhypophosphite ion such as ammonium hypophosphite (AHP). As is known inthe art, the electroless deposition process can be activated with usinga borane or using palladium displacement (e.g., using a PdCl₂ activator)for example. The cobalt or other electroless deposited metal may bedoped with various dopants or other additives as discussed below.

In one approach, the capping layer is formed using a two-phase method,which involves forming a metal nucleation layer on the etched metalportions of the substrate and forming a bulk metal layer on the metalnucleation layer. The two phases of deposition process take place atdifferent times, although some overlap is possible and even preferablein some embodiments. The two-phase method is described in further detailin the TWO-PHASE PLATING OF COBALT BARRIER LAYERS section of thispatent.

In preferred embodiments, the capping layer comprises a refractory metalsuch as cobalt, although any conductive material can be used. Thecapping layer may also contain other materials that may be impurities orpurposefully added components such as tungsten, boron, phosphorus,titanium, tantalum, zinc, cadmium, molybdenum and/or lead. Theseadditional materials may form an alloy with the metal. Or they may serveas dopants in the metal. Or they may form a non-equilibrium mixture withthe metal. Preferably, the additional materials fill or “stuff” themetal grain boundaries with amorphous material and thereby block naturaldiffusion paths. This of course improves the barrier properties of themetal capping layer. The metal (with or without such additionalmaterial) may exist in various morphologies such as amorphous orpolycrystalline morphologies. Generally, metal layers with greateramorphous character serve as more effective diffusion barriers.

The capping layer is deposited such that it is thick enough to act as adiffusion barrier but not too thick as to create too much resistancebetween conductive metal layers. To some extent, a suitable thickness ofthe capping layer is dependent upon the morphology of the underlyingmetal layer. That is, if the underlying etched metal has considerablegrain structures a thicker capping layer may be necessary compared to ifthe underlying metal grain size is small or amorphous. A preferablecapping layer thickness typically ranges between about 30 and 500Angstroms, more preferably between about 100 and 200 Angstroms.

Returning to FIG. 2, after the conductive capping layer is deposited,next an optional post-plating treatment is performed 209. Thepost-deposition process may be an anneal process, for example, in whichthe dopants from the nucleation layer and the bulk layer are permittedto intermix. For example, in the case of some embodiments where a cobaltcapping layer is deposited, boron from the cobalt nucleation layer andphosphorus from the bulk cobalt layer can mix to form a CoB_(x)P_(y)barrier capping layer. The degree of the boron/phosphorus mixing and thedistribution of the boron and phosphorus in the final CoB_(x)P_(y)capping layer will depend upon anneal process conditions (e.g.,temperature and anneal time). In addition, the microstructure of theresulting mixed CoB_(x)P_(y) capping layer can be controlled by theanneal process. Preferred substrate temperatures of anneal processes arebetween about 150 and 400 degrees Celsius. The anneal time may rangebetween about 30 seconds (high temperature) and one hour (lowtemperature, typically in a batch mode), with longer times beingrequired for lower temperatures, and is typically performed undervacuum.

After the post clean/anneal process, the next process operation is anoptional nitridation of the metal capping layer (see FIG. 2, block 211).In certain cases, it may be desirable to further enhance the barrierproperties of the metal capping layer. In these cases, this optionalnitridation process can be performed. In cases where a cobalt cappinglayer is used, nitridation produces a cobalt nitride layer that has goodbarrier properties. The cobalt nitride layer may include BN_(x) and/orPN_(x) and/or WN_(x), depending on the reducing agents/dopants used inthe previous electroless deposition steps, and preferably has anamorphous microstructure. For a detailed description of this nitridationprocess, see U.S. application Ser. No. 10/317,373, which is fullyincorporated by reference herein.

Experimental Data

FIG. 3 is a graph showing the data of the leakage currents from twodifferent integrated circuit devices. Line 301 shows data from a waferthat was fabricated using conventional cobalt capping layer methods andline 303 shows data from a wafer that was fabricated by recessing thecopper lines before depositing a cobalt capping layer in accordance withthe present invention. The individual points within the lines 301 and303 represent an individual die within the wafers that is tested forleakage current. The X axis of the graphs represents the leakage currentfor the device at 40 volts and the Y axis represents the cumulativeprobability of all the dies that are tested across the wafer. Bothwafers are fabricated using the same mask pattern. As shown by the shiftin the X direction of graph, the wafer fabricated using the cappinglayer methods of the present invention (line 303) shows reduced deviceleakage current compared to the wafer fabricated using conventionalmethods (line 301).

Two-Phase Plating of Cobalt Barrier Layers

As indicated, embodiments of the present invention provide a cobaltcapping layer using electroless deposition techniques. In one platingmethod, the cobalt capping layer is formed by first electrolesslydepositing a cobalt nucleation layer followed by an electrolesslydeposition of a bulk cobalt layer. The depositions of these layers occurin different electroless baths with differing chemical compositions.

As used herein, “cobalt” refers to chemically pure cobalt as well asmaterials largely composed of the element cobalt but also containing anyof a number of additional materials. These additional materials may beimpurities or purposefully added components such as tungsten, boron,phosphorus, titanium, tantalum, zinc, cadmium, molybdenum and/or lead.These additional materials may form an alloy with the cobalt. In caseswere the materials is microcrystalline, the added materials may resideat or near the grain boundaries or regions of bond strain having theeffect of impeding copper diffusion. Or they may serve as dopants in thecobalt. Or they may form a non-equilibrium mixture with the cobalt(i.e., a metastable material). Preferably, the additional materials fillor “stuff” the cobalt metal grain boundaries with amorphous material andthereby block natural diffusion paths. This of course improves thebarrier properties of the cobalt capping layer. The cobalt (with orwithout such additional material) may exist in various morphologies suchas amorphous, microcrystalineor polycrystalline morphologies. Generally,cobalt layers with greater amorphous character serve as more effectivediffusion barriers.

The first phase of a cobalt deposition process involves forming anucleation layer. The process conditions (including bath composition)should promote deposition of cobalt on a copper surface (or moregenerally deposition of one metal on a different metal surface). Thenucleation layer reaction kinetics should compare favorably to thekinetics achieved with conventional hypophosphite electroless platingcompositions. In addition, the electroless deposition reaction shouldselectively deposit cobalt on exposed metal regions, but not ondielectric regions.

In addition to the kinetics and selectivity constraints, the materialbeing deposited during the nucleation phase of the process should notdisplace the copper in the substrate. In other words, the nucleationlayer deposition reaction should not be a displacementreaction—particularly not one that is corrosive. The copper atoms shouldremain in place and merely provide a “substrate” for the reduced cappinglayer metal atoms. Note that in conventional hypophosphite electrolessplating, a palladium activator is used, which displaces copper duringdeposition. As indicated, this may result in local pitting and increasedsurface roughness, which can lead to electromigration problems.Palladium is also expensive, which is an additional factor of using theconventional hypophosphite electroless plating process that should betaken into account.

The second phase of the deposition process is the formation of a bulklayer over the nucleation layer. This bulk deposition process should beautocatalytic; i.e., the kinetics should favor capping layer metaldeposition on a solid capping layer metal surface. In addition, theprocess should employ relatively inexpensive reactants such ashypophophite (e.g. ammonium hypophosphite). Further, the process shouldproceed rapidly and produce a high quality barrier film.

The two phases of the capping layer deposition process take place atdifferent times, although some overlap is possible and even preferablein some embodiments. In the first phase, performed at an early time, thesubstrate is contacted with a first plating solution that deposits thenucleation layer. In the second phase, performed at a later time, thesubstrate is contacted with a second plating solution that deposits theremainder of the cobalt layer. The first and second plating solutionsmay be provided from separate reservoirs, or they may be prepared “onthe fly” during the plating operation. Further, the substrate may becontinuously contacted with a plating solution having a compositionvaries in time. During the first phase, the electroless plating solutionhas the first composition and then prior to or during the second phase,the composition changes to that of the second plating solution. Thecomposition may vary abruptly or gradually, during a short period oftime or continuously. A thin film cell may be employed to implement thisembodiment (see e.g., U.S. patent application Ser. No. 10/609,518, filedJun. 30, 2003, titled “Chemical Liquid Reaction Treatment Using ThinLiquid Layer,” by Mayer et al., which is incorporated herein byreference for all purposes). Such apparatus will be described below.

Reference will now be made to one embodiment for the formation of acobalt capping layer. Note that although the processes described belowspecifically refer to the formation of a cobalt capping layer, theprocesses may also be applied to the formation of other metal cappinglayers such as those that include tungsten, lead, cadmium, tantalum,nickel, titanium, molybdenum, and combinations and alloys thereof.

Example Process for Forming Cobalt Capping Layer

A process for the formation of a cobalt capping layer employed inaccordance with one embodiment of this invention is illustrated in theflowchart of FIG. 4. FIGS. 5A-5F provide cross-sectional views of apartially fabricated IC 500 at some of the different phases of theformation of the cobalt capping layer as described in the flowchart ofFIG. 4. Each of the process blocks of FIG. 4 will now be described indetail with reference to accompanying cross-sectional view of FIGS.5A-5F.

Referring specifically to the flowchart of FIG. 4, the process beginswith a planarized substrate having a substantially flat surface withregions of exposed copper (or other metal). Preferably, though notnecessarily, the substrate is a partially fabricated integrated circuit.In a Damascene process, for example, the copper is provided in the formof conductive lines surrounded by supporting dielectric. The flatsurface of the substrate is produced by planarization using, forexample, a CMP process or other suitable planarization technique.

As depicted in process block 401, the operation begins by annealing tochange the morphology of the copper lines layer of substrate.Specifically, the anneal operation is a thermal treatment to stabilizethe crystal structure of the copper so as to prevent the copper fromexperiencing significant mechanical stress in subsequent IC thermalprocesses. Note that this anneal process is optional in that it is notnecessary to create the cobalt capping layer. Preferable wafertemperatures of thermal anneal processes are between about 150 and 400degrees Celsius. Temperatures in the higher end of this range aregenerally preferred, but may be unattainable for some dielectricmaterials that are not able to handle thermal stress. The anneal timemay range between about 20 seconds and 2 hours, with longer times beingrequired for lower temperatures, and is typically performed undervacuum.

FIG. 5A illustrates a cross-sectional view of a partially fabricated IC500 with a metalization layer formed on an underlying region of asubstrate 505. The metalization layer includes a copper line 501 andsurrounding dielectric material 503. Note that in this case thesubstrate underlying region 505 can be a layer of active devices or anyone of a number of underlying metalization layers created as part of anIC stack. FIG. 5A shows the partially fabricated IC after the surfacehas been planarized and the copper has been suitably annealed,corresponding to process block 401 of FIG. 4.

Referring again to FIG. 4, the next operation after providing anannealed and planarized wafer is a conformal etchback of the exposedportion of the copper lines to form slight recesses in the surface ofthe wafer where the copper lines exist. See block 403. Like the annealoperation, the etchback is an optional operation. One goal of anetchback process is to provide room for the cobalt capping layer whichwill be deposited over the copper lines so that, at the end of thecobalt plating step, the surface is again planar. It is also a goal ofthe etching step to provide a sufficiently roughened copper surface topromote better cobalt adhesion. Any suitable etch process can beimplemented. In one example, the etchant is a wet chemical etchant suchas an aqueous solution of peroxide and glycine. Other useful etchantsinclude nitric acid or persulfate solutions as described in U.S. Pat.No. 5,486,234 to Contolini et. al. The process should remove copperconformally, without affecting the dielectric significantly. Preferablythe amount of copper removed is sufficient to create recesses of about 5to 50 nanometers below the dielectric field regions. FIG. 5B illustrateda cross-sectional view of partially fabricated IC 500 after it has beenconformally etched 507 at the surface of copper line 501. For a furtherdiscussion of appropriate etching conditions, see U.S. patentapplication Ser. No. 10/690,084, previously incorporated by reference.

Referring again to FIG. 4, the next operation after conformal etchbackis removal of oxide from the surface of the copper lines (see processblock 405). When the wafer is exposed to water and air in the ambientafter the etchback process, the copper surfaces can readily oxidizeforming a copper oxide film. Prior to deposition of the cobalt-cappinglayer, the copper oxide is preferably removed by chemical reduction topromote adhesion and optimize the quality of the cobalt layer. This isfrequently performed with an acidic aqueous solution, typically having apH of about 5 or lower, more preferably about 4 or lower. A dilutesulfuric acid has been found to work well. To address the potentialproblem of copper ions redistributing into the dielectric during thecleaning process, the cleaning solution preferably includes complexingagent such as EDTA.

As represented by process block 407, the next operation after copperoxide removal is electroless deposition of a cobalt nucleation layer. Acobalt nucleation layer (sometimes referred to as a “seed” layer) istypically a very thin layer of cobalt that preferably forms conformallyover the copper surfaces with minimal coverage over the dielectricsurfaces. Deposition of the cobalt nucleation layer takes place prior todeposition of the bulk cobalt layer. In addition, the depositionproceeds relatively easily (starts without significant delay, with a lowactivation energy process) and selectively on copper—in comparison tothe bulk deposition process. Also the cobalt nucleation layer and bulkcobalt film may have differing compositions due to the differingcompositions of their electroless deposition baths.

Initiation and formation of the seed layer is preferably rapid (e.g.,requiring less than about 10 seconds). As an example, the average growthrate is typically between about 2 and 8 angstroms per second. This maybe set as necessary to control the total amount of boron or other dopantin the cobalt film. Note that all boron or other dopant in the film(which is composed of both seed and bulk layer) may originate in theseed. Typically, the thickness of the nucleation layer (seed) is lessthan about 50 angstroms, and preferably between about 5 and 10 angstroms(i.e. only a few monolayers). If electrical contact the one or more ofthe plating feature can be made, progress of the process can bemonitored using a reference electrode to measure the potential thatindicates when a complete layer of cobalt has been deposited.

In many cases, the nucleation process is a self-limiting process in thatit proceeds most rapidly on a copper surface. When the copper is coveredwith the cobalt nucleation layer, the deposition rate may slow andthereby limit the final thickness of the cobalt nucleation layer. Thisis the case when certain borane containing reducing agents such as DMABare used, because the oxidation kinetics of the reducing agent aregreater on the copper substrate than the cobalt-like nucleation film.Reducing agents with these characteristics are therefore preferred overones without this property of the reverse behavior.

Preferably, the bath and process conditions are selected to yield aconformal cobalt (with boron) nucleation layer having a very small graincrystal structure or amorphous morphology. Because the film is so thin,a material with two-dimensional disorder and little correlation with thegrain structure of the underlying copper line are desired. By nucleatingwith high density of nucleation (growth center) sites, one obtains acontinuous deposit, substantially free of microvoids.

The concentration of boron in the film can be adjusted by theelectroless deposition conditions. Most fundamentally, the ratio ofcobalt ion to borane containing compound affects the boronconcentration. However, other parameters have an impact as well. Theseinclude the concentration and type of complexing agent, the bathtemperature and the bath flow rates. Preferred boron concentrationsrange between about 1 and 8% (atomic), more preferably between about 2and 4% (atomic).

Generally, the nucleation deposition bath is an aqueous solutionincluding a source of cobalt ions and a reducing agent (preferably aborane compound such as dimethyl amine borane or morpholine borane). Thebath may include one or more other components such as a stabilizer (acatalytic poison to maintain the thermodynamically unstable bath, suchas lead or cadmium), a complexing agent (prevents too much free metalion), a buffer (to keep pH range narrow), a pH adjustor, and/or one ormore surfactants.

There are many possible cobalt ion sources for use in the nucleationlayer electroless plating bath. In most cases, these are soluble cobaltsalts such as CoCl₂ (cobalt II chloride) and CoSO₄ (cobalt II sulfate),as well as other cobalt compounds that will be apparent to those ofskill in the art. Typical concentrations of cobalt ions in the aqueousplating bath range between about 10 and 50 grams/Liter in aqueoussolution, depending on the particular reagent(s) chosen, other chemicalspecies in the electroless bath, and the electroless plating conditions.

As mentioned, the reducing agent is preferably a borane compound.Suitable borane containing reducing agents include N,N-dimethylamineborane (DMAB), as well as boron hydride, hydrazine or dibutylamineborane, morpholine borane, borane-tert-butylamine, borane-ammoniacomplex (BH₃NH₃), alkali and tetramethylamine boranes (e.g. NaBH₄) andother —BH₃ containing complexes and/or derivatives. and other boranecontaining reducing agents as know in the art. If DMAB is used, typicalconcentrations range between about 1 and 20 grams/Liter, and morepreferably between about 3 and 5 grams/Liter in aqueous solution. Otherreducing agents not listed can be used, with the primary requisite thatthey have sufficient catalytic activity on copper to initiate cobaltplating on the copper surface. Preferably DMAB or other borane reducingagent is substantially the only reducing agent used in the nucleationelectroless plating bath. The boron from the reducing agent isincorporated into the growing nucleation layer.

The complexing agent can be any agent that complexes cobalt ions anddoes not significantly interfere with the deposition reaction. Somecomplexing agents are also pH buffers. Examples of suitable complexingagents include ammonia, ammonium ion compounds (e.g., ammoniumchloride), citrates (including citric acid monohydrate), glycine, EDTA,and maleates. The complexing agent concentration will differ dependingupon the complexing agent used. If, for example, citric acid, ammoniachloride or glycine are used, typical concentrations range between about10 and 80 grams/Liter in aqueous solution. The pH of the electrolessplating solution is nominally kept basic for optimal plating conditions.As indicated, a separate pH adjustor can also be included in theelectroless bath. The pH adjuster is optionally added to adjust the pH.Preferably, the pH of the electroless bath solution is maintainedbetween about 9 and 10, preferably between about 9.2 and 9.8. Oneexample of a suitable pH adjuster is tetramethyl ammonium hydroxide(TMAH).

If used, surfactants can serve to modify grain structure, improvewetting, improve solution stability, and help displace evolved hydrogengas. Examples of the suitable surfactants include PEG, PPG, tritonX-100, RE610, and the like. In one specific embodiment, polyethyleneglycol serves as a surfactant. The “Triton” surfactants available fromRohm and Haas of Philadelphia, Pa. and RE610 available from RhonePoulenc of Cedex France have been found work as suitable surfactants.The concentration of polyethylene glycol in solution ranges up to about1000 ppm, more preferably between about 100 and 500 ppm. Generally, thesurfactant should be added in an amount sufficient to meet the desiredgoals (e.g. good wetting, solution stability, etc.).

Some examples of preferred baths for nucleation are presented here.

Cobalt 5 g/L COCl₂*6H₂O

Buffer/Complexing agent 10 g/L Citric acid*H₂O

pH Adjuster: TMAH added until pH is between 9.25 and 9.80

Reducing Agent: DMAB 3 g/L

Temperature 55° C.

Cobalt 5 g/L COCl₂*6H₂O

Buffer/Complexing agent 8 g/L glycine

pH Adjuster: TMAH added until pH is between 9 and 9.25

Reducing agent: DMAB 4 g/L

Temperature 50° C.

Cobalt 5 g/L COCl₂*6H₂O

Buffer/Complexing agent 8 g/L ammonium chloride

pH Adjuster: Added until pH is between 9 and 9.25

Reducing agent: DMAB 5 g/L

Temperature 50° C.

The bath temperature for deposition of the nucleation layer preferablyranges between about 20 and 90 degrees Celsius, and more preferablybetween about 45 and 70 degrees Celsius. Other suitable electroless bathconditions and reagents can be found in, for example, the book“Electroless Plating: Fundamentals and Applications,” Glenn O. Malloryet al. editors, American Electroplaters and Surface Finishers Society,publisher (1990), which is incorporated herein by reference for allpurposes.

The cobalt nucleation layer can be deposited using any one of a numberof methods, including, for example, dipping (immersion), spraying thewafer with reactants, or use of a thin film reactor (as described inU.S. patent application Ser. No. 10/609,518, previously incorporated byreference). These methods typically involve heating the electroless bathand/or substrate to appropriate deposition temperatures. In preferredmethods, after the wafer surface has been initially exposed to theliquid electroless plating reactants, little or no convection is used inorder to accelerate nucleation of the cobalt film. It is believed thatlimiting convection promotes the formation of the borane reactionproducts at the copper surface, which initiate the nucleation process.Minimizing the fluid convection allows these products to remain on thecopper surface, thereby improving nucleation.

Referring to FIG. 5C, a thin cobalt nucleation layer 509 shown to havebeen deposited conformally and selectively over the exposed copper line501 with minimal or no coverage on the exposed surface of dielectricmaterial 503.

Referring again to FIG. 4, the next operation after deposition of thecobalt nucleation layer is transitioning to a different electrolessbath; this one for bulk cobalt deposition. Preferably, the transition isaccomplished without exposure of the wafer to ambient conditions. Seeblock 409. Avoidance of air exposure between contact with two baths ispreferred because it reduces the chance that deleterious impurities willform on or near the nucleation layer. A surface oxide, for example, canform near the newly formed cobalt nucleation film upon exposure to air,thereby hindering, and in some cases preventing, the growth of the bulkcobalt layer. It is believed that this may be due to the surface oxideinterfering with the oxidation of the reducing agent used for bulkcobalt deposition.

Prevention from air exposure can be accomplished by any of a variety ofdifferent techniques. For example, one may keep the surface of thesubstrate wet during transfer between baths. Water, buffer, platingsolution, or other solution may be maintained on the substrate surfaceduring transfer in order to protect the exposed metal. Alternatively,the transfer can be conducted under a vacuum or in an inert atmosphere.In another case, the transition is achieved by changing the electrolessbath composition from that of a cobalt nucleation bath to that of a bulkcobalt deposition bath without moving the substrate. Various reactorconfigurations allow for this. Generally, the plating bath chemistrymust be controllable by replacing nucleation layer bath with bulk layerbath using an appropriate flow system. In a preferred embodiment, thereactor provides for periodic (rather than continuous) flow through areaction compartment. The bath composition may change abruptly orgradually in the reaction compartment. A thin film reactor apparatus asdescribed below, for example, can accommodate this approach.

After the wafer has been properly transferred to a bulk cobaltdeposition electroless bath, the next process is an electrolessdeposition of a bulk cobalt barrier layer, which serves as the remainderof the cobalt capping layer 411. The bulk cobalt layer acts as the mainbarrier for preventing copper diffusion and is generally thicker thanthe cobalt nucleation layer. Generally, the bulk deposited cobalt layeris between about 2 and 10 times thicker than the nucleation layer.

Preferably, the bulk deposition process proceeds at a relatively rapidrate, on the order of 5 to 20 angstroms/second. In some cases, the bulkdeposition process is an auto-catalytic process in that the depositionreaction proceeds most rapidly on a cobalt metal layer. In such cases,the bulk deposition process is not a self-limiting process, unlike somecobalt nucleation deposition processes.

As with the cobalt nucleation layer deposition, the bulk cobaltelectroless plating bath will generally include a source of cobalt ionsand a reducing agent provided in an aqueous solution. One preferredreducing agent is hypophosphite ion (H₂PO₄ ⁻). It provides anauto-catalytic deposition process in which the cobalt deposits quiterapidly. Preferred baths contain little if any borane reducing agent.For example, it is preferred that the bath not contain more than about 1gram/Liter of DMAB.

The bulk cobalt layer may contain a dopant or combination of dopants toprovide an amorphous diffusion blocking “fill” in the cobaltmicrostructure grain boundaries as discussed previously. Suitabledopants include, for example, phosphorus, boron, tungsten, manganese andtitanium. If phosphorus is the dopant, preferred phosphorusconcentrations in the bulk cobalt layer range between about 2 and 10atomic %, more preferably between about 4 and 8 atomic %.

The choice of reducing agent used will sometimes depend upon the desireddopant. If, for example, the desired dopant is phosphorus, hypophosphiteion is a good choice for the reducing agent. It is possible to depositmore than one type of dopant, for example both phosphorus and boron,together in this process. Obviously this requires that the electrolessplating bath include both phosphorus and boron species. If phosphorusand boron are both used as dopants, preferred boron concentrations inthe bulk cobalt layer range between about 2 and 10 atomic % andpreferred phosphorus concentrations range between about 4 and 8 atomic%.

In the case of tungsten additives, one example of a suitable platingbath additive is ammonium tungstate ((NH₄)₂WO₄). The amount of tungstenwithin the cobalt film is preferably between about 0.5% and 10%(atomic), more preferably, between about 1% and 5% (atomic), dependingupon process condition. In addition, using a combination of thesereducing agents and dopant additives can create a variety of dopedcobalt film such combinations such as CoB_(x)P_(y), CoB_(x)W_(z) andCoPyW_(z) and CoB_(x)P_(y)W_(z) (where x, y and z are small fractionsless than about 0.1)

Other additives such as complexing agents, pH adjustors, buffers,stabilizers, and surfactants may be included in the bulk cobalt platingbath. See Mallory et al. (page 499 et seq.), previously incorporated byreference.

Examples of bulk deposition bath compositions follow.

30 g/L Cobalt Sulfate, 70 g/L Citric acid (anhydrous), 60 g/L ammoniumchloride, 20 g/L Ammonium hypophosphite (AHP), pH adjusted to 9 withTMAH, temperature 75-85° C.

35 g/L Cobalt Sulfate, 35 g/L Citric acid (anhydrous), 70 g/L ammoniumsulfate, 40 g/L Ammonium hypophosphite (AHP), pH adjusted to 9.3 withTMAH, temperature 60-70° C.

30 g/L Cobalt Chloride, 100 g/L Citric acid (anhydrous), 50 g/L ammoniumchloride, 30 g/L Ammonium hypophosphite, (AHP), 2 g/L (NH₄)₂WO₄, pHadjusted to 9.5 with TMAH, temperature 80-90° C.

The above baths are only a limited set of examples, and suitable bathcan be found in the literature and from those skilled in the art. Aswith the cobalt nucleation layer deposition, any one of a number ofmethods may be used to deposit the bulk cobalt layer, including, forexample, dipping (immersion), spraying the wafer with reactants or useof a thin film reactor.

Referring to FIG. 5D, partially fabricated IC 500 now contains a bulkcobalt layer 511 formed over the cobalt nucleation layer 509 withminimal coverage over the surface of dielectric 503. Typical thicknessesfor a bulk cobalt layer range between about 50 and 450 Angstroms.

As represented by process block 413 in FIG. 4, the next operation afterbulk cobalt layer deposition is an optional post plating treatmentand/or anneal. A variety of post plating treatments may be performedafter the electroplating process including rinsing, ultrasonic(megasonic) cleaning, scrubbing (as typically performed, for example,after CMP processes), edge and back of wafer processing (e.g., edgebevel removal (EBR) and/or backside wafer cleaning), and anneal. Anynumber of these post anneal process can be preformed as needed. Edge andback of wafer processing, particularly EBR, are described in U.S. Pat.No. 6,309,981 issued Oct. 30, 2001 to Mayer et al., which isincorporated herein by reference for all purposes.

In some preferred processes, a post-deposition anneal process is used.In these anneal processes the dopants from the nucleation layer and thebulk layer are permitted to intermix. For example, boron from the cobaltnucleation layer and phosphorus from the bulk cobalt layer can mix toform a CoB_(x)P_(y) barrier capping layer. The composition of theCoB_(x)P_(y) capping layer will depend upon the boron and phosphorusconcentrations and thicknesses of the previously deposited cobaltnucleation and bulk cobalt layers. The degree of the boron/phosphorusmixing and the distribution of the boron and phosphorus in the finalCoB_(x)P_(y) capping layer will depend upon anneal process conditions(e.g., temperature and anneal time). For example, the boron may be moreconcentrated at the copper-cobalt interface while the phosphorus may bemore concentrated in regions above the interface in the bulk regions. Inaddition, the microstructure of the resulting mixed CoB_(x)P_(y) cappinglayer can be controlled by the anneal process.

Preferred substrate temperatures of post anneal processes are betweenabout 200 and 450 degrees Celsius. The anneal time may range betweenabout 20 seconds and 4 hours, with longer times being required for lowertemperatures, and is typically performed under a reducing atmosphere(e.g. Nitrogen/hydrogen) or a vacuum. FIG. 5E provides a cross-sectionalview of the partially fabricated IC 500 after a post-deposition annealprocess. After anneal, the components of the cobalt nucleation layer 509and the bulk cobalt layer 511 of FIG. 5D can intermix to produce a morehomogenous overall barrier layer 513. As indicated, a more homogeneousbarrier layer may have a CoB_(x)P_(y) composition. The final thicknessof layer 513 will depend upon the thicknesses of the cobalt nucleationand cobalt bulk layer thicknesses. Typical final cobalt capping layerthicknesses range between about 50 and 500 Angstroms. The composition ofthe final cobalt capping layer 513 typically ranges between about 3 and8 atomic % phosphorus and between about 0.1 to 6 atomic % boron. If atungsten dopant is employed, it too may be provided in a concentrationsof between about 0.1 and 2 atomic %.

After the post clean/anneal process, the next process operation is anoptional nitridation of the cobalt capping layer (see FIG. 4, block415). In certain cases, it may be desirable to further enhance thebarrier properties of the cobalt capping layer. In these cases, thisoptional nitridation process can be performed. For a detaileddescription of this nitridation process, see U.S. application Ser. No.:10/317,373, which is fully incorporated herein.

In short, the nitridation process involves exposing the cobalt cappinglayer to a plasma which contains nitrogen species (for example, anammonia composition or a mixture of hydrogen and nitrogen), therebycreating a cobalt nitride layer. Some techniques involve placing thesemiconductor substrate on a RF electrode and exposing the cobaltsurface to a nitrogen containing plasma. Suitable nitrogen containinggases to create the plasma include N₂, NH₃ and N₂H₄, for example. Insome techniques, a RF electrode is located away from the substrate. Insome techniques, a high-density plasma (HDP) is used.

Nitridation produces a cobalt nitride layer that has good barrierproperties. The cobalt nitride layer may include BN_(x) and/or PN_(x)and/or WN_(x), depending on the reducing agents/dopants used in theprevious electroless deposition steps, and preferably has an amorphousmicrostructure.

FIG. 5F illustrates a cross-sectional view of the partially fabricatedIC 500 after a nitridation process. A thin cobalt nitride layer 515 isformed over the cobalt capping layer 513. The cobalt nitride layer 515is preferably between about 20 angstroms to 400 angstroms thick, morepreferably between about 20 angstroms to 200 angstroms thick (even morepreferably between about 50 and 100 angstroms), depending on thenitridation and cobalt plating process conditions. The cobalt nitridelayer 515 contains preferably between about 0.1% and 20% (atomic), andmore preferably between about 0.1% and 5% (atomic) of nitrogen,depending upon process conditions.

Apparatus

As indicated, the electroless deposition processes can be performed bydipping (immersion), spraying, etc. Suitable apparatus for performingthese processes will be well known to those of skill in the art.Examples of such apparatus are described in U.S. patent application Ser.No. 09/996,425, filed Nov. 27, 2001 by Andryuschenko et al. and titled“Electroless Copper Deposition Method for Preparing Copper Seed Layers,”U.S. patent application Ser. No. 10/235,420, filed Sep. 3, 2002 by Parket al. and titled “Electroless Layer Planting Process And Apparatus,”U.S. patent application Ser. No. 10/274,837, filed Oct. 18, 2002 byMinshall et al. and titled “Electroless Copper Deposition Apparatus,”and U.S. patent application Ser. No. 10/272,693, filed Oct. 15, 2002 byMayer et al. and titled “Methods And Apparatus For Airflow and HeatManagement In Electroless Plating.” Each of these references isincorporated herein by reference in its entirety and for all purposes.

As indicated previously, a particularly preferred apparatus forconducting the deposition processes of this invention is a thin filmreactor such as that described in U.S. Provisional Patent ApplicationNo. 60/392,203, now U.S. patent application Ser. No. 10/609,518, filedJun. 30, 2003 by Mayer et. al and titled “Chemical Liquid ReactionTreatment Using Thin Liquid Layer”, previously incorporated byreference. FIGS. 6A and 6B show one example of a thin film reactorsuitable for use with this invention.

Referring to FIG. 6A, a reactor vessel or module contains an outer wall301 composed of a suitable material capable of withstanding the typicaloperating temperature of the plating or etching operation. It alsoshould be resistant to chemical attack from the fluid reactants that itwill be exposed to during operation. Examples of suitable materialsinclude PVC, PVDF, PTFE, PE, PP. The wafer 602 sits on a wafer chuck 603which includes a rotary shaft 604 connected to a motor (not shown)sitting below the containment vessel's bottom 601 a. The chuck includes(for example) three or more support pins 605 for holding the wafer abovethe chuck arms 605 a. Alignment pins 606 are useful in centering thewafer during its inserting in the module from the wafer handling robotarm, and can be tapped to facilitate this operation. It also serves thefunction of containing the wafer from spinning out during subsequentoperations occurring with rotation (pre-wetting, thin film plating oretching, rinsing, and high speed drying). Design of a chuck for use inEBR operations has been described, for example, in U.S. Pat. No.6,537,416, entitled “Wafer Chuck for Use in Edge Bevel Removal of CopperFrom Silicon Wafers”, issued Mar. 25, 2003. A waste drain 607 is locatedat the bottom of the containment vessel in 601 a (the containment vesselis defined by the wall 601 and bottom 601 a. Rinse waste and materialnot captured for recycle of reactant chemical is captured in this drainafter falling off the walls of the vessel. The base of the containmentvessel 601 a can be sloped 608 to facilitate draining.

In a particularly preferred configuration, the reactor contains areactant recycling diversion apparatus 609. This apparatus can consistof one or more troughs 610 located radially outside the wafer. Thesetroughs can be raised or lowered to substantially align with the waferplan so as to collect fluid emanating from the wafer in a radialdirection (which is induced in large measure, by the rotation of thewafer surface). Connected to each of these troughs is a separate drainhole 611. The trough is preferably designed such that fluid is directeddownward and into this hole (i.e. it is at the lowest location in thetrough). The holes lead to the primary drain tube 612. In a preferredembodiment, the primary drain tube 312 fits inside a secondary draintube 613 with a slightly larger diameter. The primary drain tube canmove down into the secondary drain tube and have enough travel to alwaysstay inside the secondary tube over its normal length of operationaltravel. The secondary tube is in fluid communication with the reactantliquid source (not shown). The reactant liquid source may contain a heatexchanger for cooling (to stop auto-catalytic reactions) or heating(preparing the chemical for subsequent recycling and reaction). Thechoice of these heating or cooling operations should be determined bythe properties of the particular materials/chemicals being used, theirstability and processing temperature. Chemistries that are highlyunstable should be cool, and heated right before application to thewafer, pumped from the containment vessel, heated in line, flowed into afluid gap through a heated treatment head, and recycled back to a cooledcontainment vessel via the reactant recycling diversion apparatus 609.More stable chemistries might be maintained at the operating temperaturein the reactant containment vessel. In both of these cases, a “bleed andfeed” of the fluid in the reactant vessel can be employed to avoidsubstantial changes to bath properties due to consumption of reactantsin the process and auto-decomposition. Determination of the optimumliquid-source size and turnover rate is made according to reactantsolution stability measurements, consistent with the liquid-sourceturnover time (i.e., removal rate (liter/hr)/source volume (liter)).Removal rate should include considerations of evaporation, consumption,decomposition, and collection efficiency of reactant in the recyclingdiversion apparatus.

The reactor may include a nozzle 614 or similar applicator of DI(deionized water), for depositing a thin film of water 602 a on thewafer surface. It is preferred that this nozzle be located at theperiphery of the module and spray inward onto the surface. The DI watercan be used to pre-wet the wafer (removing all air and entrapped bubblesfrom the wafer) and to rinse the wafer surface from chemical left afterdeposition (either nucleation or bulk layer) or removal operationsconducted with liquid-layer treatment head 617. Heated DI water can beused to improve the efficiency of these processes, and to minimizeheating times when hot reactant fluids are used subsequently. A rinsenozzle directed at the backside of the wafer (not shown) for removingany incidental exposure of the surface to processing fluids can also beincluded. Heating the wafer from both the top and bottom with hot DIwater can prepare the wafer for heated processing, improving throughput.

FIG. 6B shows the same layout as in FIG. 6A, but with the liquid-layertreatment head 617 in the engaged position over the wafer. The reactorhead comprises a significant mass of a highly conducting material with aheat capacity substantially greater than that of the substrate.Generally, the (total, not specific) heat capacity of the head should begreater than 10 times that of the wafer, and the thermal conductivity ofthe material of the heating mass in the head should as large as possibleand generally greater than 0.2 Watt cm⁻¹ K⁻¹. Examples of suitablematerials are materials, which are metals at around ambient temperatures(particularly aluminum and copper). Because the material for the heatingmass of the head may not be compatible with the reacting fluids (e.g.,the electroless solution may had a tendency to plate onto the reactinghead metal), the bottom surface of the head may be covered or coatedwith a thin film of a compatible material (not shown), such as PVDF, PE,PP, or PTFE coating. The film should be sufficiently thick to becontinuous and to protect the head from spurious reaction and breakingunder handling and typical operation, but also sufficiently thin so asto the film does not substantially reduce the heat transferring abilityof the head to the wafer surface (via the thin liquid layer of reactantsin the fluid gap between the head and the substrate wafer). Similarly,the port and path of injected liquid through the head is not exposed tothe head thermal mass if it is susceptible to attack to avoid prematuredecomposition. A tube 619 made of a suitable material (e.g. plastic)carries the liquid to the treating surface of the head and to the fluidgap between the head and the wafer. In a particularly preferredembodiment, the liquid is directed to a number of exiting fluid outletholes whose location and density are selected to improve and optimizethe uniformity of the deposition reaction.

In one aspect, the treatment head is heated and maintained at anelevated temperature by one or a plurality of means. In another aspect,an electrical heating element 620 is attached to the top of, or embeddedinto, the treatment head. The temperature can be controlled by aregulator that senses the unit's temperature via thermocouple,thermistor, or similar device embedded in the bulk of the head.Alternatively, a heat exchange manifold with a tortuous fluid bath caninterface with flow of an externally heated fluid.

The heating head rotates with the wafer, opposite to the wafer, or isstationary. Rotation enables further modification and control of thehydrodynamics and mass transfer of reactants to the wafer surface in thethin liquid layer created between the wafer and the head. The fluid gapbetween the wafer and the head can be maintained by means similar tothose known in the construction of fluidic barring. In such anembodiment, the gap is self-regulating and enables a minimum, narrowspacing. The fluid gap size is dependent on the relative rotation rate,injected fluid flow rate, and shape of the heating head surface.Alternatively, the fluid gap can be controlled by mechanical stops 621and the like, as depicted in FIG. 6B. The gap can be fine-tuned with aturn-screw against a hard stop, with three-position contact to a supporttied to the same base as the chuck mount of the wafer. As depicted inFIG. 6B (in contrast to FIG. 6A), the reactant recycling diversionapparatus 609 is in a lowered position and liquid emanating from thewafer/head fluid gap flies into the collection trough.

Preferably, the wafer can be exposed first to the nucleation solution bymixing pre-heated reactants containing all components less theactivating borane compound with ambient temperature concentrated borane.The mixing occurs just prior to application of the fluid to the thinfilm reactor gap and wafer surface, and heats the borane containingfluid to approximately the desired nucleation plating temperature anddilutes it to the appropriate concentration. After filling the head withreactant fluid, the flow is stopped and, with the heating head of thereactor at the operating temperature, the temperature quickly approachesthe desire wafer interface temperature. Following the relatively brief(10 to 40 seconds) nucleation period, the nucleation plating solution isremoved from the gap by flowing growth-phase bath material (similarlymixed just prior to flooding the reactor gap), stopping flow, andallowing the fluid to contact the wafer for the appropriate bulk filmgrowth period (typically 40 to 120 seconds). In some cases, where thedesired plating temperature for the nucleation and bulk films aredifferent, fluid is flushed from the reactor with water and air afterforming the nucleation film, and either the wafer is moved to anotherstation with a different head set point temperature, of the waferremains in the chuck and a different head is moved over the wafer. Ineither case, a large range of different bath processing conditions (incomposition and temperature) can be achieved using a minimal amount ofplating solution using this approach.

Compositions for Isotropic Etching

Etching of copper-containing materials can be accomplished isotropicallywith the use of particular compositions containing an oxidizing agentand a complexing agent. It was unexpectedly discovered that the natureof complexing agent is of particular importance when isotropic uniformetching without significant pitting or roughening is desired. Etchingcompositions that provide high etch rates (e.g., at least about 1,000Å/minute, preferably at least about 2,000 Å/minute) in the pH range ofbetween about 5-12, preferably between about 6-10 were developed.Significantly, etching occurs at substantially the same rates within therecessed features (or copper-filled lines) of different sizes, —that is,it is isotropic. Further, different surfaces within the recessedfeatures are etched at substantially the same rates, e.g., etching atthe corners of the formed recessed features occurs at the same rate asetching at the feature bottoms. Etching with the described compositionsalso occurs uniformly across the wafer, with little etch rate variationbetween the central portions and the edges of the wafer.

In contrast, conventional copper etching compositions, e.g., etchingcompositions having low pH of less than about 5, commonly exhibitnon-isotropic behavior, with etch rates within smaller features beingsubstantially greater than etch rates within larger features. Further,high pitting and surface roughness are observed with the use ofconventional etching compositions.

The unusual isotropic behavior of the etches described herein isattributed to the rate-limiting reaction occurring at the coppersurface. Without being bound by a particular theory, it is possible thatin the pH range of between about 6-12, copper oxide is being formed atthe copper surface and is immediately solubilized and removed by thecomplexing agent of the copper etching composition. It is noted, thatadvantageously, etching compositions in some embodiments describedherein do not form a layer of copper oxide resident on copper surface,but instead afford a smooth oxide-free copper surface having highreflectivity (e.g., reflectivity greater than 120% after 5,000 Å etch).Thus, if any oxide is being formed during the etching reaction, it isimmediately removed in situ, such that an additional oxide-removingoperation is not required. It is understood, however, that in otherembodiments isotropic etching compositions may form a layer of oxideresident on the surface, which may be removed in a subsequent separateoperation.

The etching compositions that will be described in detail below and inthe attached appendix afford isotropic copper etching. The etching isnon-grain specific, e.g., it does not occur at substantially higherrates at grain boundaries, and therefore does not result in undesiredfaceting. The etch rate is feature size and pitch independent. Further,they provide reduced pitting and surface roughness and afford smoothoxide-free surfaces having high reflectivity.

According to one embodiment the compositions include an oxidizing agent(e.g., a peroxide, a persulfate, a permanganate, etc.) and a bi-dentate,tri-dentate or quadridentate complexing agent having at least one aminogroup. The other coordinating group or groups may be, e.g., acarboxylate and/or another amino groups. Hydrogen peroxide is apreferred oxidizing agent in some embodiments, due to its highsolubility and low cost.

The nature of complexing agent was found to be of particular importance.For example, simple unidentate ligands, such as ammonia, and largecarboxylate-rich multidentate ligands, such as ethylene diaminetetraacetic acid (EDTA) were found to afford low etch rates and surfaceoxide formation.

Unexpectedly, many aminoacids and diamines, such as glycine andethylenediamine (EDA) were found to afford excellent etch rates,isotropic behavior, low surface roughness, and no residual surfaceoxide.

“Aminoacids” as used herein include both biologically occurring(natural) and unnatural aminoacids, and refer to molecules having atleast one carboxylic group and at least one amino group. Aminoacids maybe optionally substituted with a variety of substituents. In a preferredembodiment, aminoacids include a non-substituted primary aminogroup,although in other embodiments, aminoacids may be N-derivatized. Glycine,DL-alanine, beta-alanine, serine, DL-methionine, and DL-valine werefound to be suitable complexing agents for isotropic etches. Preliminaryresults for aminoacids having bulkier substituents, such as leucine,glutamine, aspartic acid, tyrosine, cystine, and N-methylated aminoacidsarcosine showed that etch rates were low.

Examples of typical etching compositions comprising aminoacids includecompositions having a pH of between about 6-10, an aminoacid (e.g.,glycine) and H₂O₂. Highest etch rates were observed at H₂O₂:glycinemolar ratio of about 1:2. Buffering agents, corrosion inhibitors, and pHadjustors (such as tetramethylammonium hydroxide) can be added to thecompositions as needed. In some embodiments it is preferable to conductetching above room temperature (e.g., at about 45° C.) to achievereduced pitting and improved etch rate. It is noted that thecompositions described herein are wet etching compositions and aredistinct from CMP slurries in that they do not contain abrasiveparticles.

In other embodiments, diaimines, triamines, tetramines and otherpolyamines are used as complexing agents. These can be derivatized atnitrogen (e.g., can be N-alkyl substituted) or at other positions.Examples include ethylenediamine (EDA, H₂NCH₂CH₂NH₂),N-methylethylenediamine (CH₃NHCH₂CH₂NH₂), diethylenetriamine(H₂NCH₂CH₂NHCH₂CH₂NH₂), and tris-2-aminomethylamine (tren,N(CH₂CH₂NH₂)₃).

Advantageously, diamine-containing etching compositions afford very highetch rates and lead to smooth metal surfaces having very highreflectivity. Further, diamines often do not require expensive pHadjustors to reach the desired pH. Examples of typical etchingcompositions comprising polyamines include compositions having a pH ofbetween about 6-10, a polyamine (e.g., EDA) and H₂O₂.

Another advantageous feature of the present compositions is that etchingcan be rapidly quenched at high pH. For example, if etching needs to bestopped, a pH adjuster may be supplied into the system to increase thepH of the etchant to, e.g., greater than about 10-12. The etch rate forthe described etching compositions decreases at high pH, and etching canbe shut down at high pH by introduction of a suitable pH adjuster (e.g.,an OH-containing adjustor).

The described etching compositions can be used for forming recesses incopper lines in the presence of exposed dielectric according to methodsdescribed above. Further, the described compositions can be used forselective etching of copper in the presence of diffusion barriermaterial, according to methods described in U.S. patent application Ser.No. 11/251,353 naming Reid et al. as inventors, filed on Oct. 13, 2005.

It is understood, that compositions for isotropic etching describedherein and are not limited to the context of capping applications, andcan be employed in any application where isotropic etching of copper isdesired. While the described compositions and methods are particularlyadvantageous for use during fabrication of semiconductor devices havingcopper-filled features with widths of less than about 400 nm, they canalso be used in a wide variety of applications beyond IC fabrication.

Experimental details of provided etching compositions and methods aredescribed in the experimental section presented below.

EXPERIMENTAL Glycine/H₂O₂ Etching

Dependence of copper etch rates on the concentrations of glycine andH₂O₂ in the etching solution was investigated. FIG. 7 is a plotillustrating etch rate dependence on glycine concentrations at variousconcentrations of H₂O₂. Glycine concentration was varied from about 0g/L to about 20 g/L. H₂O₂ concentration was varied from about 0 g/L toabout 320 g/L. It can be seen that even at glycine concentration of 5g/L, etch rates of greater than 1,000 Å were achieved. The peak etchrates of about 6,000 Å/minute (curve (g)) and about 11,000 Å/minute(curve (h)) were achieved at about 1:2 H₂O₂/glycine molar ratio.

FIG. 8 is a plot illustrating etch rate dependence on H₂O₂concentrations at various concentrations of glycine. Peak etch rateswere achieved at about 1:2 H₂O₂/glycine molar ratio. Above 1:2H₂O₂/glycine molar ratio surface oxidation as an intermediate begins todominate, consuming electrons and modulating the reaction rate.

FIG. 9 is a plot illustrating dependence of copper etch rate on solutiontemperature for three different etching compositions containing H₂O₂ andglycine. Etch rates at 20° C. and at 45° C. were measured. Etch ratesincrease as the solution temperature is raised. For example, in curve(a) etch rate is increased from about 2,500 Å/minute to about 18,000Å/minute; in curve (b) etch rate is increased from about 1,000 Å/minuteto about 4,000 Å/minute; and in curve (c) the etch rate is increasedfrom about 1,000 Å/minute to about 2,000 Å/minute. The best reflectivityvalues were observed for (b) and (c).

FIG. 10 is a plot illustrating dependence of copper etch rate onsolution pH for three different etching compositions containing H₂O₂ andglycine. The etch rate is decreasing when the pH of solution is raisedabove the pKa of glycine (9.6).

Roughness of copper surface obtained after etching was measured forseveral etching conditions, and is illustrated in FIG. 11.Reflectivities of greater than 120% (measured at 480 nm, after 4,500 Åof copper was removed) were observed for an etching chemistry containing5 g/L glycine and 320 g/L H₂O₂, wherein the etching solution temperaturewas 45° C. Reflectivities were generally lower for solutions containinglower H₂O₂/glycine ratios.

Aminoacid/H₂O₂ Etching

A number of aminoacids were tested as complexing agents in etchingcompositions containing an aminoacid and hydrogen peroxide. In additionto glycine, appreciable etching rates were exhibited by DL-alanine,β-alanine, DL-valine, serine, and DL-methionine. Leucine, glutamine,aspartic acid, tyrosine, cystine, anthranilic acid, sarcosine,3-aminobenzoic acid and 4-aminobenzoic acid exhibited significantlylower etch rates. Compositions containing cistine, anthranilic acid,3-aminobenzoic acid and 4-aminobenzoic acid did not appreciably etchcopper.

FIG. 12 is a plot illustrating pH dependence of copper etch rates for anumber of aminoacids. Glycine, L-gluthamine, leucine, β-alanine,DL-alanine, serine, aspartic acid, DL-valine, and methionine weretested. The concentrations of components in the etching solutions wereidentical: 1M H₂O₂ and 0.066 M aminoacid.

A Comparison of Ligands in Copper Etching

The following solutions were tested for their ability to etch copperisotropically:

-   (a) Ethylenediamine (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This solution exhibits a relatively high etch rate of 4600 Å/min (e.g.compared to the aminoacids described above), and also exhibitedexcellent isotropic behavior. Reflectivity of copper surface was 131%after etching (134% prior to etching) per 6900 Å removed.

-   (b) Glycine (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This solution exhibits an etch rate of 1580 Å/min and isotropicbehavior. Reflectivity of copper surface was 140% after etching (134%prior to etching) per 1053 Å removed. In this example it is notable thatthe reflectivity, indicative of the surfaces micro-smoothness, increasedfollowing the etching.

-   (c) Ammonium Persulfate (0.066M), No Hydrogen Peroxide, pH 2.6, 20°    C.

This low pH (acidic) etching solution exhibits an etch rate of 1849Å/min, however copper surface with high roughness was obtained afteretching. Reflectivity of copper surface was only 61% after etching (134%prior to etching) per 1849 Å removed. A matte finish was observed.

-   (d) Ammonium Acetate (0.066M), H₂O₂ (1.06 M), pH 3.6, 20° C.

This low pH etching solution exhibits an etch rate of 1163 Å/min,however a red/brown copper oxide surface film was obtained during andafter etching. Reflectivity of copper surface was only 8.8% afteretching (134% prior to etching) per 1163 Å removed.

-   (e) Ammonium Persulfate (0.066M), No Hydrogen Peroxide, pH 8.9, 20°    C.

This etching solution exhibits an etch rate of only 613 Å/min, howevercopper surface with red/brown oxide was obtained after etching.Reflectivity of copper surface was 52% after etching (134% prior toetching) per 429 Å removed.

-   (f) Ammonium Hydroxide (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This etching solution exhibits an etch rate of only 53 Å/min, howevercopper surface with red/brown oxide was obtained after etching.Reflectivity of copper surface was 68% after etching (134% prior toetching) per 53 Å removed.

-   (g) EDTA (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This etching solution exhibits an etch rate of only 5 Å/min.Reflectivity of copper surface was 135% after etching (134% prior toetching) per 5 Å removed, indicating very little interaction with thesurface (i.e. little metal removal or film formation).

-   (h) Citric Acid (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This etching solution also exhibits no appreciable etching at all.

-   (i) Oxalic Acid (0.066M), H₂O₂ (1.06 M), pH 8.9, 20° C.

This etching solution also exhibits no appreciable etching at all.

-   (j) Ammonium Acetate (0.066M), H₂O₂ (1.06 M), pH 9.6, 20° C.

This etching solution exhibits an etch rate of 429 Å/min, however coppersurface with red/brown oxide was obtained after etching. Reflectivity ofcopper surface was 13% after etching (134% prior to etching) per 429 Åremoved.

A bar graph illustrating dependence of etch rates on etching chemistryis shown in FIG. 13.

It can be seen that of the examples given, only EDA and glycine provideboth high etching rates and isotropic behavior. Ammonium hydroxide andammonium salts lead to formation of oxide deposits on copper surfaceand/or to high surface roughness. EDTA, citric and oxalic acids do notexhibit appreciable etch rates.

Etching Solutions Containing EDA and H₂O₂

A copper film was etched by spraying an etching solution containing EDA(4 g/L, 0.067 M), and H₂O₂ (120 g/L of 30% H₂O₂, or about 1.9 M) ontothe substrate. The pH of solution was 8.9; the temperature was 20° C.The EDA solution and hydrogen peroxide solution were mixed in linesprior to delivery of the resulting etchant onto the substrate. Etch rateof about 4,600 Å/minute was observed. After 7,000 Å of copper wereremoved, the reflectivity was 122% (compared to 125% reflectivity priorto etching). FIG. 14 illustrates a dependence of the amount of etchedcopper on the spray etch time. It can be seen that about 9,000 Å wereremoved in about 120 seconds. The amount of etched copper linearlydepends on the spray etch time.

In another experiment pH dependence of copper etch rate was studied. Anetching solution containing 2 g/L (0.033 M) EDA and 30 g/L of 30% H₂O₂(9 g/L, or 0.47M) was used. pH was adjusted with H₂SO₄ or TMAH dependingon the need to increase of decrease the pH from the un-modifiedEDA/peroxide mixture value. It was observed that at pH values lower than7, copper surface became rough and was generally non-uniform inappearance. At high pH (above 11) the etch rates declined though thesurface appear relatively unaltered (reflective/smooth). It was foundthat the preferred pH for EDA etch was from about 8 to about 11. Thedependence of the etch rate on pH is illustrated in FIG. 15.

FIG. 16 illustrates etch rate dependence on the concentration of H₂O₂for a solution containing EDA (0.067M) and H₂O₂ (10-40 g/L) at a fixedpH of 8.9. The etch rate is moderately increasing as the amount of H₂O₂is increased.

FIG. 17 illustrates etch rate dependence on the concentration of EDA fora solution containing EDA (0-8 g/L) and H₂O₂ (1 M) at pH 8.9. It can beseen that etch rate is strongly dependent on the concentration of EDA.The rate of etching increase in a near linear fashion for concentrationgreater than 8 g/L, for example, at a rate of about 2.2 um/min at aconcentration of 32 g/L (data not shown in FIG. 15).

Table 1 illustrates changes in reflectivity (per 1000 Å of copperetched) for different compositions of an etching solution containing EDAand hydrogen peroxide.

TABLE 1 Copper surface roughness obtained after etching with differentsolutions comprising EDA. EDA Concentration H2O2 % Change (g/L)Concentration (g/L) pH Reflectance/(1000 Å) 1 39.6 8.9 11.1 2 9.9 7 5.42 9.9 8.1 3.9 2 9.9 8.9 2.8 2 9.9 10.3 14.4 2 9.9 11.6 14.2 4 9.9 8.91.4 4 19.8 8.9 0.67 4 39.6 8.9 0.34 8 39.6 8.9 2.8

In order to determine change in reflectivity and to assess surfaceroughening, reflectivity of copper layer was measured before and afteretching, and the change in the number is normalized to 1,000 Å of copperetched. For example, if prior to etching copper reflectance is R1=135%,and the reflectance decreases to R2=120% after D=4200 Å of copper hasbeen removed, the change in reflectance is calculated as[(R2−R1)/(R1×D)]×100%×1000=2.6% (per 1,000 Å removed). It can be seenthat provided etching with provided etching solutions resulted inreflectivity changes of less than about 15% per 1,000 Å of copperetched.

Etching Solutions with Other Complexing Agents Containing an Aminogroup

A number of solutions containing different complexing agents weretested. pH was adjusted to 8.9 or 8.8 with TMAH or H₂SO₄.

-   (a) N-methylethylenediamine (0.066M), H₂O₂ (1.00 M), pH 8.9, 20° C.

This solution exhibits an appreciable etch rate of 1575 Å/min andisotropic behavior. Reflectivity of copper surface was 127% afteretching (134% prior to etching) per 1575 Å removed.

-   (b) Sarcosine (0.066M), H₂O₂ (1.00 M), pH 8.9, 20° C.

This solution exhibits an etch rate of 11.5 Å/min.

-   (c) Taurine (0.066M), H₂O₂ (1.00 M), pH 8.9, 20° C.

This solution exhibits an etch rate of 12 Å/min.

-   (d) Ethanolamine (0.066M), H₂O₂ (1.00 M), pH 8.9, 20° C.

This solution does not exhibit appreciable etching.

FIG. 18 is a bar graph illustrating etch rates for the compositionscontaining complexing agents listed above.

1. A method of isotropically etching copper-containing portions of apartially fabricated semiconductor substrate containing a region ofcopper-containing metal, the method comprising: (a) receiving thepartially fabricated semiconductor substrate comprising an exposedregion of copper-containing metal; and (b) isotropically etching thecopper-containing metal on the substrate by contacting thecopper-containing metal on the substrate with an abrasive-free wetetching solution at a pH in a range of between about 5 and 12, whereinthe solution comprises (i) at least one bidentate, tridentate, orquadridentate complexing agent for ions of copper, wherein saidcomplexing agent is selected from the group consisting of a diamine, atriamine, a tetramine, and an aminoacid and (ii) an oxidizer, whereincontacting the substrate with the etching solution is selected from thegroup consisting of spraying the etching solution onto the rotatingsemiconductor substrate, spin on contact, and contacting thesemiconductor substrate with the etching solution in a thin filmreactor.
 2. The method of claim 1, wherein the complexing agentcomprises an aminoacid.
 3. The method of claim 1, wherein the complexingagent comprises glycine.
 4. The method of claim 1, wherein thecomplexing agent comprises a polyamine.
 5. The method of claim 1,wherein the complexing agent comprises a diamine.
 6. The method of claim1, wherein the complexing agent comprises ethylene diamine.
 7. Themethod of claim 1, wherein the complexing agent comprisesN-methylethylenediamine.
 8. The method of claim 1, wherein thecomplexing agent comprises a triamine.
 9. The method of claim 1, whereinthe complexing agent comprises a tetraamine.
 10. The method of claim 1,wherein the isotropic etching occurs without forming a visible copperoxide film on the etched surface.
 11. The method of claim 1, wherein theisotropic etching is performed at a pH range of between about 6 and 10.12. The method of claim 1, wherein the oxidizing agent is hydrogenperoxide.
 13. The method of claim 1, wherein the complexing agent isselected from the group consisting of alanine, beta-alanine, serine,methionine and valine.
 14. The method of claim 1, wherein the complexingagent comprises diethylenetriamine (H₂NCH₂CH₂NHCH₂CH₂NH₂).
 15. Themethod of claim 1, wherein the complexing agent comprisestris(2-aminoethyl)amine N(CH₂CH₂NH₂)₃.
 16. The method of claim 1,wherein: the complexing agent comprises at least one selected from thegroup consisting of a diamine, a triamine or a tetramine; and theoxidizing agent comprises hydrogen peroxide.
 17. The method of claim 1,wherein the etching comprises at least partially removing copperoverburden from the semiconductor substrate.
 18. The method of claim 1,further comprising stopping the isotropic etching process by increasingthe pH of the wet etching solution.
 19. The method of claim 18, whereinincreasing the pH of the wet etching solution comprises adding a basicpH-adjustor to the wet etching solution.
 20. The method of claim 1,wherein the wet etching solution further comprises a pH adjustor. 21.The method of claim 20, wherein the complexing agent comprises apolyamine and the pH adjustor comprises sulfuric acid.
 22. The method ofclaim 1, wherein (b) comprises isotropically etching thecopper-containing metal at a rate of at least about 1,000 Å a minute.23. The method of claim 22, wherein isotropically etching thecopper-containing metal comprises etching the material at substantiallythe same rates on different surfaces of a recessed feature.
 24. Themethod of claim 22, wherein the isotropic etching rate of thecopper-containing metal is substantially uniform across the partiallyfabricated semiconductor substrate.